The present invention relates to a method of detecting image density in an electrophotographic copying machine and an apparatus for carrying out the method.
In an electrophotographic copying machine, control of toner density or concentration in a developer comprising toner particles and carrier particles is required to produce a copied image having a constant and predetermined image density. For this purpose, there have been proposed various methods of measuring the density of the toner image formed on a photosensitive member in accordance with the copying operation. In one method, a patch having a preselected optical density is attached to a copy board of the copying apparatus, the board comprising a member or plate on which an original to be copied is placed during exposure to light. A toner image of the patch is formed at a preselected region on a drum having a photo-sensitive member or layer, the density of the patch toner image then being measured by use of a sensor for detecting the density.
However, this density measuring method exhibits drawbacks in controlling the toner image density of the original to be copied. For example, the sensor inherently exhibits uneven sensitivity, and additionally is not stable with respect to its operating characteristics on changes of temperature. Further, because the sensor is usually located at an intermediate position between the developing station and the cleaning station in the vicinity of the photo-sensitive surface of the drum, the sensor is very susceptible to contamination by the toner particles. For these reasons, the output signal produced by the sensor does not represent the actual toner density with a reasonable accuracy. In other words, although the density of the patch toner image may actually remain unchanged, the output signal voltage of the sensor will be progressively lowered as though the toner density or concentration were decreased. In particular, in the case of a copying machine in which the sensor is also used for detecting the possible occurrence of jamming of the copy sheets, the sensor is subjected to considerable contamination by the toner particles since the sensor is disposed near to the cleaning station having cleaning means such as a brush. Thus, an erroneous density signal is more likely to be generated. Moreover, variations in the surface conditions of the photo-sensitive drum will exert adverse influences on the output signal of the sensor, providing obstacles in performing an accurate measurement of toner density.
As attempts to eliminate such disadvantages as are described above, it has been heretofore known to adjust the individual sensors, readjust the sensors for every exchange of the photo-sensitive drum, cyclically clean the sensor and/or provide a temperature compensation means in the measuring circuit. However, with all of the measures described above, there still exists difficulty in satisfactorily correcting the density signal. In particular, it is impossible to effect required corrections at the right times.
The present invention teaches sensing of the optical density of the surface of a photo-sensitive drum at a blank region in which no toner image is formed preferably in addition to the detection of a patch toner image density. When the patch toner image density is detected, both the densities are constantly compared with each other, to thereby effect control of constant toner density by compensating in a satisfactory manner any error components appearing in the output signal from the sensors; such errors are ascribable to unevenness in sensitivity of the sensor, variations in the sensor characteristics as a function of temperature, contaminations by toner particles, variations in the surface condition of the photo-sensitive drum, or like factors.